Light guide plate made of lead-free glass having a high refractive index and image display device using a light guide plate

ABSTRACT

Provided is a light guide plate for an image display device which uses lead-free glass, has excellent color reproducibility and a light weight, and may obtain a wide viewing angle. A light guide plate for an image display device, which guides image light inputted from an image display element and outputs the image light toward a user&#39;s pupil, is configured to be made of lead-free glass having a refractive index of 1.8 or more with respect to a wavelength of the image light, and to have internal transmittance of 0.6 or more with respect to a wavelength of 400 nm when a plate thickness is 10 mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/033,080, filed on Jul. 11, 2018, and allowed on Apr. 1, 2019, whichclaims the benefit of Japanese Patent Application Nos. 2017-136277 and2018-127421, and filed on Jul. 12, 2017, and Jul. 4, 2018, respectively,the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a light guide plate used for an imagedisplay device such as a head-mounted display, and an image displaydevice using the same.

BACKGROUND ART

Recently, a head-mounted display has been provided for practical use asa device that enlarges a two-dimensional image by using a virtual imageoptical system and displays the enlarged virtual image to allow anobserver to observe the enlarged virtual image.

The head-mounted displays are classified into a see-through transmissivehead-mounted display and a non-transmissive head-mounted display.Because the transmissive head-mounted display is used in combinationwith an information terminal or used to provide augmented reality (AR),the transmissive head-mounted display needs to have a small size andexcellent portability. In addition, because the non-transmissivehead-mounted display is used to watch movies, or provide games, virtualreality (VR), or the like, the non-transmissive head-mounted displayneeds to have a wide viewing angle in which a sense of immersion isobtained.

The head-mounted display is disclosed in, for example, PatentDocument 1. The head-mounted display disclosed in Patent Document 1 isprovided with a display device which displays an image, a light guidemember to which the image displayed on a display element is inputted,and a propagation means which totally reflects the inputted image in thelight guide member to propagate the inputted image toward a user'spupils. Further, the light guide member is made of a glass materialhaving a refractive index of about 1.7 in order to obtain apredetermined viewing angle.

DOCUMENT OF RELATED ART Patent Document

[Patent Document 1]

Japanese Patent Application Laid-Open No. 2010-243787

SUMMARY OF INVENTION Technical Problem

An image having a viewing angle of about 10.0 degrees may be obtainedaccording to the light guide member of the head-mounted displaydisclosed in Patent Document 1, but there is a need for a light guidemember that have a wider viewing angle and excellent colorreproducibility for the purpose of the VR. In addition, there is a needfor a light guide member that is smaller in size and lighter in weightfor the purpose of the AR, and there is also a need for a thinner lightguide member. In addition, recently, there is also a need for alead-free light guide member in terms of environmental protection andeffective utilization of resources.

The present invention has been made in consideration of theaforementioned situations, and an object of the present invention is toprovide a light guide plate for an image display device (head-mounteddisplay, etc.) which uses lead-free glass, has excellent colorreproducibility and a light weight, and may obtain a wide viewing angle,and to provide an image display device using the light guide plate.

Solution to Problem

To achieve the aforementioned object, a light guide plate according tothe present invention is a light guide plate for an image display devicewhich guides image light inputted from an image display element andoutputs the image light toward a user's pupil, in which the light guideplate is made of lead-free glass having a refractive index of 1.8 ormore with respect to a wavelength of the image light, and has internaltransmittance of 0.6 or more with respect to a wavelength of 400 nm whena plate thickness is 10 mm.

According to the light guide plate having the aforementionedconfiguration, it is possible to implement an image display device whichis thin and light in weight and has excellent color reproducibility anda wide viewing angle.

In addition, the light guide plate may have a first surface and a secondsurface that face each other, and parallelism of the first surface andthe second surface may be 20 arc seconds or less. In addition, in thiscase, a difference between a maximum value and a minimum value of adistance from the first surface to the second surface may be 5 μm orless.

In addition, when transmitted light of a standard light source D65 whenthe plate thickness is 10 mm is represented by an x-y chromaticitydiagram, chromaticity of x may be 0.31 to 0.34, and chromaticity of ymay be 0.33 to 0.36.

In addition, the plate thickness may be 0.5 to 1.0 mm.

In addition, the light guide plate may include a first optical elementwhich diffracts the guided image light and outputs the image light tothe user's pupil. In addition, in this case, the first optical elementmay be made of a material having a refractive index higher than arefractive index of air. In addition, the first optical element may bemade of a material having a refractive index higher than a refractiveindex of the light guide plate.

In addition, the light guide plate may include a second optical elementwhich diffracts the image light inputted from the image display elementand inputs the image light to the light guide plate. In addition, inthis case, the second optical element may be made of a material having arefractive index higher than a refractive index of air. In addition, thesecond optical element may be made of a material having a refractiveindex higher than a refractive index of the light guide plate.

In addition, according to another aspect, an image display deviceaccording to the present invention includes: a light source whichoutputs illumination light; an image display element which receives theillumination light from the light source and outputs image light; andany of the light guide plates which guides the image light inputted fromthe image display element and outputs the image light toward a user'spupil.

Effect of Invention

As described above, according to the present invention, it is possibleto implement a light guide plate for an image display device which useslead-free glass, has excellent color reproducibility and a light weight,and may obtain a wide viewing angle. In addition, it is possible toimplement an image display device using the light guide plate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A and FIG. 1B are views illustrating a configuration of ahead-mounted display using a light guide plate according to an exemplaryembodiment of the present invention.

FIG. 2 is a side view schematically illustrating the configuration ofthe head-mounted display using the light guide plate according to theexemplary embodiment of the present invention.

FIG. 3A and FIG. 3B are graphs illustrating a characteristic of a lightguide plate according to Example 1 of the present invention.

FIG. 4A, FIG. 4B, and FIG. 4C are views illustrating a simulation modelof a head-mounted display using the light guide plate according toExample 1 of the present invention.

FIG. 5A and FIG. 5B are graphs illustrating a characteristic of a lightguide plate according to Example 2 of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the drawings. Further, in thedrawings, the same or similar constituent elements are designated by thesame reference numerals, and a description of thereof will be omitted.

FIG. 1 is a view illustrating a configuration of a head-mounted display1 (hereinafter, abbreviated to an “HMD 1”) using a light guide plate 10according to an exemplary embodiment of the present invention, in whichFIG. 1A is a perspective front view, and FIG. 1B is a perspective rearview. As illustrated in FIGS. 1A and 1B, a spectacle lens 3 is mountedat a front side of a spectacle type frame 2 mounted on a user's head. Abacklight 4 for illuminating an image is mounted on a mounting unit 2 aof the spectacle type frame 2. A signal processing instrument 5 forprojecting an image and speakers 6 for reproducing voice are provided ontemple portions of the spectacle type frame 2. The light guide plate 10is a thin-plate-shaped optical component having a rectangular shape, anda flexible printed circuit (FPC) 7, which constitutes wiring extendedfrom a circuit of the signal processing instrument 5, is wired along thespectacle type frame 2. A display element unit (e.g., a liquid crystaldisplay element) 20 is wired to a central position between the user'sboth eyes by the FPC 7, and the display element unit 20 is retained sothat an approximately central portion of the display element unit 20 isdisposed on an optical axis of the backlight 4. The display element unit20 is fixed relative to the light guide plate 10 so that the displayelement unit 20 is positioned at the approximately central portion ofthe light guide plate 10. In addition, at a point positioned in front ofthe user's eyes, holographic optical elements (HOEs) 32R and 32L (firstoptical elements) are closely attached and fixed to a first surface 10 aof the light guide plate 10 by adhesion or the like. HOEs 52R and 52Lare stacked on a second surface 10 b of the light guide plate 10 at aposition opposite to the display element unit 20 based on the lightguide plate 10.

FIG. 2 is a side view schematically illustrating a configuration of theHMD 1 of the present exemplary embodiment. Further, in FIG. 2, forclarity of the drawings, only main parts of the invention areillustrated, but the spectacle type frame 2 and the like are notillustrated. As illustrated in FIG. 2, the HMD 1 is structured to besymmetrical with respect to a vertical axis based on a centerline X thatconnects a center of an image display element 24 and a center of thelight guide plate 10. In addition, a light beam, which is inputted tothe light guide plate 10 from the image display element 24 and has awavelength, is split into two light beams, and the two light beams areguided to the user's right and left eyes, respectively, as describedbelow. Optical paths of the light beams, which are guided to therespective eyes and have wavelengths, are also approximately symmetricalwith respect to the vertical axis based on the centerline X.

As illustrated in FIG. 2, the backlight 4 has a laser source 21, adiffusion optical system 22, and a micro lens array 23. The displayelement unit 20 is an image forming unit having the image displayelement 24, and the display element unit 20 is operated, for example, ina field sequential manner. The laser source 21 is a laser source thatcorresponds to the respective wavelengths of R (wavelength of 436 nm), G(wavelength of 546 nm), and B (wavelength of 633 nm), and the lasersource 21 sequentially emits the light beams having the respectivewavelengths at a high speed. The light beams having the respectivewavelengths are inputted to the diffusion optical system 22 and themicro lens array 23, converted into uniform and parallel light beamshaving high directivity without light quantity irregularity, andperpendicularly inputted to a display panel surface of the image displayelement 24.

For example, the image display element 24 is a transmissive liquidcrystal (LCD T-LCOS) panel operated in a field sequential manner. Theimage display element 24 modulates the light beams having the respectivewavelengths in accordance with an image signal created by an imageengine (not illustrated) of the signal processing instrument 5. Thelight beams having the respective wavelengths, which are modulated topixels within an effective region of the image display element 24, areinputted to the light guide plate 10 while having predetermined lightbeam cross sections (having approximately the same shape as theeffective region). Further, for example, the image display element 24may be substituted with other types of display elements such as adigital mirror device (DMD) or a reflective liquid crystal (LCOS) panel,a micro electro mechanical system (MEMS), organic electro-luminescence(EL), or inorganic EL.

Further, the display element unit 20 is not limited to the displayelement operated in a field sequential manner, but an image forming unitof a synchronous display element (a display element having an RGB colorfilter with a predetermined array in front of a light output surface)may be applied. In this case, for example, a white light source may beused as the light source.

As illustrated in FIG. 2, the light beams having the respectivewavelengths, which are modulated by the image display element 24, aresequentially inputted into the light guide plate 10 from the firstsurface 10 a. The HOEs 52R and 52L (second optical elements) are stackedon the second surface 10 b of the light guide plate 10. For example,each of the HOEs 52R and 52L is a reflective volume phase type HOEhaving a rectangular shape and has a configuration in which threephotopolymers, on which interference fringes corresponding to the lightbeams having the respective wavelengths of R, G, and B are recorded, arestacked. That is, each of the HOEs 52R and 52L is configured to have awavelength selecting function of diffracting the light beams of therespective wavelengths of R, G, and B and allowing light beams havingthe other wavelengths to penetrate therethrough.

In addition, each of the HOEs 52R and 52L may be a single-layeredphotopolymer on which interference fringes corresponding to the lightbeams having the respective wavelengths of R, G, and B are recorded.

In addition, each of the HOEs 52R and 52L is configured by two-layeredphotopolymers, and the wavelength selecting function corresponding tothe light beams having the respective wavelengths of R, G, and B may beimparted. For example, it is conceivable that each of the HOEs 52R and52L is configured by two-layered photopolymers including asingle-layered photopolymer on which interference fringes correspondingto the light beams having the respective wavelengths of R and G arerecorded, and a single-layered photopolymer on which interferencefringes corresponding to the light beam having the wavelength of B arerecorded. Further, the HOEs 32R and 32L are also reflective volume phasetype HOEs and have the same layer structures as the HOEs 52R and 52L.For example, the HOEs 32R, 32L, 52R, and 52L may have approximately thesame pitches of the interference fringe patterns.

The HOEs 52R and 52L are stacked in a state in which the centers of theHOEs 52R and 52L coincide with each other and the interference fringepatterns are reversed at 180 degrees. Further, the HOEs 52R and 52L areclosely attached and fixed to the second surface 10 b of the light guideplate 10 by adhesion or the like so that the centers of the HOEs 52R and52L coincide with the centerline X in the state in which the HOEs 52Rand 52L are stacked. The light beams, which are modulated by the imagedisplay element 24 and have the respective wavelengths, are sequentiallyinputted to the HOEs 52R and 52L through the light guide plate 10

Each of the HOEs 52R and 52L diffracts the light beams, which aresequentially inputted and have the respective wavelengths, at apredetermined angle in order to guide the light beams to the right andleft eyes. The light beams, which are diffracted by the HOEs 52R and 52Land have the respective wavelengths, are repeatedly and totallyreflected at an interface between the light guide plate 10 and air,propagated into the light guide plate 10, and inputted to the HOEs 32Rand 32L. Here, the HOEs 52R and 52L impart the same diffraction angle tothe light beams having the respective wavelengths. For this reason, allof the light beams, which have the wavelengths and are inputted toapproximately the same incident position on the light guide plate 10 (orin other words, outputted from approximately the same coordinate withinthe effective region of the image display element 24), are propagatedalong approximately the same optical path in the light guide plate 10and inputted to approximately the same position on the HOEs 32R and 32L.From another point of view, the HOEs 52R and 52L diffract the lightbeams having the respective wavelengths of RGB so that a positionalrelationship between pixels within the effective region of the imagedisplayed within the effective region of the image display element 24 isfaithfully reproduced on the HOEs 32R and 32L.

In the present exemplary embodiment as described above, the HOEs 52R and52L diffract all of the light beams, which have the wavelengths andoutputted from approximately the same coordinate within the effectiveregion of the image display element 24, so that the light beams areinputted to approximately the same position on the HOEs 32R and 32L.Meanwhile, in another exemplary embodiment, the HOEs 52R and 52L may beconfigured to diffract all of the light beams, which have thewavelengths and have inherently the same pixels relatively shiftedwithin the effective region of the image display element 24, so that thelight beams are inputted to approximately the same position on the HOEs32R and 32L.

The light beams, which are inputted onto the HOEs 32R and 32L and havethe respective wavelengths, are diffracted by the HOEs 32R and 32L andsequentially outputted approximately perpendicularly to the outside fromthe second surface 10 b of the light guide plate 10. The light beams,which are outputted as approximately parallel light beams and have therespective wavelengths as described above, form images on retinas of theuser's right and left eyes as virtual images I created by the imagedisplay element 24. In addition, a condenser function may be imparted tothe HOEs 32R and 32L to allow the user to observe the virtual image I asan enlarged image. That is, the light beams may be outputted at anglesso as to approach the center of the pupil as much as light beamsinputted to the peripheral region of the HOEs 32R and 32L, and the lightbeams may form images on the user's retinas. Alternatively, to allow theuser to observe the virtual image I as the enlarged image, the HOEs 52Rand 52L may diffract the light beams having the respective wavelengthsof RGB so that the positional relationship between the pixels on theHOEs 32R and 32L has the enlarged and similar shape to the positionalrelationship between the pixels within the effective region of the imagedisplayed within the effective region of the image display element 24.

In addition, as a material for forming the HOEs 32R, 32L, 52R, and 52L,a material, which has a refractive index higher than a refractive indexof air, may be used. Since the HOEs 32R, 32L, 52R, and 52L are formed byusing this material, it is possible to effectively improve diffractionefficiency of the HOEs 32R, 32L, 52R, and 52L. In addition, thematerial, which is used to form the HOEs 32R, 32L, 52R, and 52L, mayhave a refractive index higher than a refractive index of a materialused to form the light guide plate 10. Since the HOEs 32R, 32L, 52R, and52L are formed by using this material, it is possible to improvediffraction efficiency and effectively improve clarity of images. Forexample, a material including one or more of Si₃N₄, TiO₂, Nb₂O₅, andTa₂O₅ may be used as a material used to form the HOEs 32R, 32L, 52R, and52L.

Since the light beams having the respective wavelengths of RGBsequentially form the images on the user's retinas at a high speed asdescribed above, the user recognizes the images created by the imagedisplay element 24 as color images. Further, an actual distance betweenthe user's eye and the image display element 24 is just several tens ofmillimeters. However, since the light beams having the respectivewavelengths are inputted to the eyeballs as approximately parallel lightbeams, the user may clearly and visually recognize the images created bythe image display element 24 at the time of infinite far-sightedness. Inaddition, each of the HOEs 32R and 32L, which is a reflective HOE of avolume phase type, has a small half-value width of diffractionefficiency and high light transmittance of an external image. For thisreason, the user may clearly observe the image of the external worldtogether with the image displayed on the image display element 24.

Further, the present exemplary embodiment adopts a configuration inwhich the image display element 24 is provided in the form of a singleplate without separately providing a display element for a right eye anda display element for a left eye. For this reason, an effect such as adecrease in manufacturing costs is obtained. In addition, the lightbeams from the common object point (i.e., the image of the image displayelement 24 in the form of a single plate) are guided to the user'srespective eyes via the optical path lengths which are the same in theleft and right sides. For this reason, a synchronized image may beinputted to the user's respective eyes.

In the configuration of the present exemplary embodiment as describedabove, an air converted optical path length of the light beam travelingin the light guide plate 10 becomes shorter as a refractive index isincreased, and as a result, an apparent viewing angle with respect to awidth of the image display element 24 is increased as the refractiveindex is increased. Therefore, in the present exemplary embodiment,lead-free glass, which has a high refractive index of 1.8 or more withrespect to the respective wavelengths of RGB and has a plate thicknessof 0.5 to 1.0 mm, is used for the light guide plate 10 in order toincrease the viewing angle (i.e., so that a viewing angle θv withrespect to the virtual image I of the image display element 24 is, forexample, 13.5 degrees or more). Further, the light guide plate 10 mayhave a higher refractive index, that is, 1.95 or more.

In the case in which the lead-free glass having a high refractive indexis used for the light guide plate 10, preferable composition ranges ofrespective components, which constitute the glass, are as follows.

SiO₂: 0 to 35%

B₂O₃: 0 to 55%

ZnO: 0 to 35%

Y₂O₃: 0 to 40%

ZrO₂: 0 to 30%

TiO₂: 0 to 20%

In addition, the contents of the respective components are indicated bymass % with respect to overall mass of the glass in terms of oxideconversion composition. Here, assuming that all of the oxides, thecombined salts, the metal fluorides, and the like used as raw materialsof the glass constituent components of the present invention aredecomposed and converted into oxides while being melted, the oxideconversion composition refers to the composition which represents therespective components included in the glass under a condition in whichthe total mass of the corresponding produced oxides is 100 mass %.

SiO₂ is a component that constitutes a basic structure of the glass, andwhen the SiO₂ content is equal to or higher than 0%, the stability ofthe glass is improved, and viscosity suitable to form molten glass iseasily maintained. If the SiO₂ content is higher than 35%, a refractiveindex is decreased and a liquid-phase temperature or a glass-transitiontemperature is increased, and as a result, the SiO₂ content ispreferably 0 to 35%. In addition, the SiO₂ content is more preferably 1to 25%, and much more preferably 1 to 15%.

B₂O₃ is a network forming oxide that imparts a low dispersion propertyto the glass. When the B₂O₃ content is equal to or higher than 0%, it ispossible to improve stability and chemical durability of the glass. Ifthe B₂O₃ content is higher than 55%, a refractive index is decreased,and as a result, the B₂O₃ content is preferably 0 to 55%. In addition,the B₂O₃ content is more preferably 1 to 45%, and much more preferably 1to 40%.

ZnO is a component that maintains a high refractive index and imparts alow dispersion property. When the ZnO content is equal to or higher than0%, it is possible to effectively decrease a melting temperature, aliquid-phase temperature, and a transition temperature of the glass. Ifthe ZnO content is higher than 35%, the glass is easily devitrified andchemical durability thereof deteriorates, and as a result, the ZnOcontent is preferably 0 to 35%. In addition, the ZnO content ispreferably 0.5 to 25%, and much more preferably 1 to 20%.

Y₂O₃ is a component that maintains a high refractive index and imparts alow dispersion property. When the Y₂O₃ content is equal to or higherthan 0%, it is possible to improve stability and chemical durability ofthe glass. If the Y₂O₃ content is higher than 40%, the glass is easilydevitrified and a glass-transition temperature or a strain pointtemperature is increased, and as a result, the Y₂O₃ content ispreferably 0 to 40%. In addition, the Y₂O₃ content is more preferably 0to 20%, and much more preferably 0.1 to 8%.

ZrO₂ is a component that adjusts an optical function and improvesstability and a weather-resistant property of the glass. When the ZrO₂content is equal to or higher than 0%, it is possible to improvestability of the glass. If the ZrO₂ content is higher than 30%,stability of the glass deteriorates and an increase in dispersion iscaused, and as a result, the ZrO₂ content is preferably 0 to 30%. Inaddition, the ZrO₂ content is more preferably 0.5 to 15%, and much morepreferably 1 to 10%.

TiO₂ is a component that serves as a modifier for a glass structure.When the TiO₂ content is equal to or higher than 0%, it is possible toimprove a refractive index of the glass and improve stability of theglass. If the TiO₂ content is higher than 20%, there occur an increasein dispersion, deterioration in thermal stability, an increase inliquid-phase temperature, and deterioration in coloration degree, and asa result, the TiO₂ content is preferably 0 to 20%. In addition, the TiO₂content is preferably 1 to 18%, and much more preferably 1 to 15%.

In addition, the first surface 10 a and the second surface 10 b of thelight guide plate 10 of the present exemplary embodiment may have highparallelism in order to assuredly guide the light beams having therespective wavelengths of RGB to the HOEs 32R and 32L. Therefore, thelight guide plate 10 of the present exemplary embodiment is processed sothat the parallelism of the first surface 10 a and the second surface 10b is 20 arc seconds or less. Further, the parallelism of the firstsurface 10 a and the second surface 10 b may be 10 arc seconds or less,particularly, 5 arc seconds or less. Further, if the parallelism of thefirst surface 10 a and the second surface 10 b is greater than 20 arcseconds, the light beams having the respective wavelengths of RGB cannotbe accurately guided to the HOEs 32R and 32L.

As described above, since the lead-free glass having a high refractiveindex of 1.8 or more with respect to the respective wavelengths of RGBis used for the light guide plate 10 of the present exemplaryembodiment, it is possible to configure the HMD 1 which is thin andlight in weight and has the wide viewing angle θv (e.g., 13.5 degrees ormore).

In addition, the light guide plate 10 of the present exemplaryembodiment may have high transmittance with respect to the respectivewavelengths of RGB in order to accurately guide the light beams havingthe respective wavelengths of RGB (i.e., to accurately reproducecolors). In particular, the wavelength of B is within a wavelengthregion of 400 to 500 nm, and the displayed image (virtual image I)entirely becomes yellow as the component of B is decreased, and as aresult, the light guide plate 10 may be configured so that the componentof B is not decreased. Therefore, the glass material having internaltransmittance of 0.6 or higher with respect to a wavelength of 400 nmwhen a plate thickness is 10 mm is used for the light guide plate 10 ofthe present exemplary embodiment. Further, the higher internaltransmittance is favorable, and the internal transmittance may be 0.65or higher, more particularly, 0.70 or higher.

In addition, the light guide plate 10 of the present exemplaryembodiment needs to accurately guide the light beams having therespective wavelengths of RGB (in a well-balanced manner) in order tofaithfully reproduce the color image by using the image display element24. In general, an x-y chromaticity coordinate in a visual field of astandard light source D65 at 10 degrees is x: 0.3138 and y: 0.3310, butif there is absorption in the light guide plate 10, a shift occurs fromthe coordinate, such that a color sense of the use varies. Therefore, aglass material, which has x: 0.31 to 0.34 and y: 0.33 to 0.36 whentransmitted light of the standard light source D65 when the platethickness is 10 mm is represented by the x-y chromaticity diagram(coordinate), is used for the light guide plate 10 of the presentexemplary embodiment. Further, x may be 0.31 to 0.33, more particularly,0.31 to 0.32. In addition, y may be 0.33 to 0.35, more particularly,0.33 to 0.34. Further, if x exceeds the range of x: 0.31 to 0.34 or if yexceeds the range of y: 0.33 to 0.36, coloration of the displayed image(virtual image I) is increased, and the user observes the displayedimage (virtual image I) having greatly different color senses.

In addition, the light guide plate 10 of the present exemplaryembodiment may have high flatness in order to accurately guide the lightbeams having the respective wavelengths of RGB and obtain an image whichis less blurred. Therefore, the light guide plate 10 of the presentexemplary embodiment is polished so that a difference (i.e., a totalthickness variation (TTV)) between a maximum value and a minimum valueof a thickness of the light guide plate 10 (i.e., a distance between thefirst surface 10 a and the second surface 10 b) is 5 μm or less.Further, the flatness (i.e., TTV) may be 2 μm or less, moreparticularly, 1 μm or less. Further, if the TTV exceeds 5 μm, thedisplayed image (virtual image I) observed by the user is very blurry.

EXAMPLES

Next, specific Examples of the light guide plate 10 will be described.Further, the light guide plate 10 of the present invention is notlimited to the present Examples.

Example 1

“TAFD55”, a product manufactured by HOYA Corporation, was used as aglass material, and the light guide plate 10 was obtained by processingthe glass material so that the light guide plate 10 has length 50mm×width 20 mm×thickness 1.0 mm. Main properties thereof are as follows.

Refractive index (@436 nm): 2.04600

Refractive index (@546 nm): 2.00912

Refractive index (@633 nm): 1.99406

Parallelism of the first surface 10 a and the second surface 10 b: 15arc seconds

Flatness (TTV): 5 μm

Internal transmittance with respect to a wavelength of 400 nm when aplate thickness is 10 mm: 0.749

Chromaticity of transmitted light of the standard light source D65 whena plate thickness is 10 mm: x=0.3187, y=0.3385

In addition, the composition of the “TAFD55” is as follows.

SiO₂: 1 to 10%

B₂O₃: 1 to 10%

ZnO: 0 to 10%

Y₂O₃: 0 to 1%

ZrO₂: 1 to 10%

In addition, FIG. 3 is a graph illustrating the properties of thepresent Example, in which FIG. 3A illustrates spectral characteristicswhen the plate thickness is 10 mm, and FIG. 3B is a chromaticity diagramof the transmitted light of the standard light source D65 when the platethickness is 10 mm.

As a result of embedding the light guide plate 10 obtained as describedabove in the HMD 1, and then evaluating an image at a position of an eyepoint, an image having a wide viewing angle, high brightness, and highcontrast could be observed.

Table 1 is a table illustrating the respective parameters and resultsthereof (i.e., viewing angle θv) at the time of simulating the viewingangle when the light guide plate 10 of the present Example was embeddedin the HMD 1. In addition, FIG. 4 is a view illustrating a simulationmodel, and the respective parameters illustrated in FIG. 4 correspond tothe parameters in Table 1.

TABLE 1 Diffraction angle of the HOEs 52R and 52L: θ′_(D) 85.0 Thicknessof the light guide plate 10: t 1.0 Number of times of reflection in thelight guide plate 10 15 Width of the HOEs 52R and 52L 5.2 Refractiveindex of the HOEs 52R, 52L, 32R, and 32L 1.80 Refractive index of thelight guide plate 10 2.04600 Angle of light beam in the light guideplate 10: θ′i 61.2 Shift amount per one reflection: t/tanθ′i 1.8 L (=t/tanθ′i × (number of times of reflection + 1)) 27.3 Optical path lengthper one reflection: t/cosθ′i 2.1 Actual optical path length (= t/cosθ′i× (number of 31.1 times of reflection + 1)) Air converted optical pathlength 15.2 Additional optical path length (distance to a pupil 5.0 fromthe HOEs 32R and 32L) Viewing angle θv 14.7

As illustrated in FIG. 4, in the present simulation model, the lightbeams having the respective wavelengths of RGB (broken lines in FIG. 4)are inputted to the HOEs 52R and 52L, diffracted at a diffraction angleθ′D (85.0°) in the HOEs 52R and 52L, and inputted to the light guideplate 10 at a light beam angle θ′i (61.2°) (FIG. 4A). Further, the lightbeams, which are inputted to the light guide plate 10 and have therespective wavelengths of RGB, are reflected a predetermined number oftime (15 times) in the light guide plate 10 (thickness: t=1.0 mm,refractive index: 2.04600), and outputted toward the user's pupils (FIG.4B). Further, the viewing angle θv in this case is determined by apositional relationship between a pupil position P and the HOEs 52R and52L, and as illustrated in FIG. 4C, the viewing angle θv may beindicated by an angle formed between the pupil position P and both endsof the HOEs 52R and 52L which are disposed at positions spaced apartfrom each other by the air converted optical path length+the additionaloptical path length from the pupil position P (distance to the pupilfrom the HOEs 32R and 32L). Further, the viewing angle θv of the HMD 1embedded with the light guide plate 10 of the present Example was 14.7degrees when calculating the viewing angle θv by using the respectiveparameters in Table 1 (see Table 1). As described above, it can be seenthat a wider viewing angle is obtained according to the light guideplate 10 of the present Example in comparison with the HMD having aconfiguration in the related art.

Example 2

“TAFD65”, a product manufactured by HOYA Corporation, was used as aglass material, and the light guide plate 10 was obtained by processingthe glass material so that the light guide plate 10 has length 50mm×width 20 mm×thickness 1.0 mm. Main properties thereof are as follows.

Refractive index (@436 nm): 2.10226

Refractive index (@546 nm): 2.06011

Refractive index (@633 nm): 2.04305

Parallelism of the first surface 10 a and the second surface 10 b: 15arc seconds

Flatness (TTV): 5 μm

Internal transmittance with respect to a wavelength of 400 nm when aplate thickness is 10 mm: 0.609

Chromaticity of transmitted light of the standard light source D65 whena plate thickness is 10 mm: x=0.3227, y=0.3440

In addition, the composition of the “TAFD65” is as follows.

SiO₂: 1 to 10%

B₂O₃: 1 to 10%

ZnO: 0 to 1%

Y₂O₃: 0 to 1%

ZrO₂: 1 to 10%

TiO₂: 10 to 20%

In addition, FIG. 5 is a graph illustrating the properties of thepresent Example, in which FIG. 5A illustrates spectral characteristicswhen the plate thickness is 10 mm, and FIG. 5B is a chromaticity diagramof the transmitted light of the standard light source D65 when the platethickness is 10 mm.

As a result of embedding the light guide plate 10 obtained as describedabove in the HMD 1, and then evaluating an image at a position of an eyepoint, an image having a wide viewing angle, high brightness, and highcontrast could be observed.

Table 2 is a table illustrating the respective parameters and resultsthereof (i.e., viewing angle θv) at the time of simulating the viewingangle when the light guide plate 10 of the present Example was embeddedin the HMD 1. Further, a simulation model is identical to thatillustrated in FIG. 4.

TABLE 2 Diffraction angle of the HOEs 52R and 52L: θ′_(D) 85.0 Thicknessof the light guide plate 10: t 1.0 Number of times of reflection in thelight guide plate 10 15 Width of the HOEs 52R and 52L 5.2 Refractiveindex of the HOEs 52R, 52L, 32R, and 32L 1.80 Refractive index of thelight guide plate 10 2.10226 Angle of light beam in the light guideplate 10: θ′i 58.5 Shift amount per one reflection: t/tanθ′i 1.6 L (=t/tanθ′i × (number of times of reflection + 1)) 24.5 Optical path lengthper one reflection: t/cosθ′i 1.9 Actual optical path length (= t/cosθ′i× (number of 28.7 times of reflection + 1)) Air converted optical pathlength 13.7 Additional optical path length (distance to a pupil from 5.0the HOEs 32R and 32L) Viewing angle θv 15.9

Similar to the simulation in Table 1, the viewing angle θv of the HMD 1embedded with the light guide plate 10 of the present Example was 15.9degrees when performing the simulation by using the respectiveparameters in Table 2. As described above, it can be seen that a widerviewing angle is obtained according to the light guide plate 10 of thepresent Example in comparison with the HMD having a configuration in therelated art.

The exemplary embodiments of the present invention have been describedabove. The present invention is not limited to the configurations of therespective exemplary embodiments and the specific numerical values, butmay be variously modified within the scope of the technical spirit ofthe present invention.

The HOEs 32R, 32L, 52R, and 52L are, for example, transmissive HOEs. Inthis case, for example, the HOEs 52R and 52L are closely attached andfixed to the first surface 10 a of the light guide plate 10 that facesthe display element unit 20 in the state in which the HOEs 52R and 52Lare stacked on the first surface 10 a, and the HOEs 52R and 52L diffractthe light beams having the respective wavelengths from the displayelement unit 20 so that the light beams are totally reflected in thelight guide plate 10 and propagated to the HOE 32R or the HOE 32L. Forexample, the HOEs 32R and 32L are closely attached and fixed to thesecond surface 10 b of the light guide plate 10 that faces the user'spupil, and diffract the light beams, which are propagated in the lightguide plate 10 and have the respective wavelengths, toward the user'spupil.

In addition, the HOEs 32R, 32L, 52R, and 52L in the exemplary embodimentare described as being made of photopolymers, but the present inventionis not limited to this configuration, and for example, the HOEs 32R,32L, 52R, and 52L may be formed by depositing optical thin films ontothe surface of the light guide plate 10.

In addition, for example, the laser source 21 of the display elementunit 20 may be an LED or an LD (semiconductor laser) backlight thatsequentially emits the light beams having the respective wavelengths ofR, G, and B at a high speed.

In addition, the configuration in which the display element unit 20 isdisposed at the side of the first surface 10 a of the light guide plate10 and the HOEs 52R and 52L are disposed at the side of the secondsurface 10 b of the light guide plate 10 has been described in thepresent exemplary embodiment, but the present invention is not limitedthereto, and for example, the display element unit 20 may be disposed atthe side of the second surface 10 b of the light guide plate 10, and theHOEs 52R and 52L may be disposed at the side of the first surface 10 aof the light guide plate 10.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1: HMD    -   2: spectacle type frame    -   2 a: mounting unit    -   3: spectacle lens    -   4: backlight    -   5: signal processing instrument    -   6: speaker    -   7: FPC    -   10: light guide plate    -   10 a: first surface    -   10 b: second surface    -   20: display element unit    -   21: laser source    -   22: diffusion optical system    -   23: micro lens array    -   24: image display element    -   32R, 32L, 52R, 52L: HOE

The invention claimed is:
 1. A light guide plate for an image displaydevice which guides image light inputted from an image display elementand outputs the image light toward a user's pupil, wherein, the lightguide plate is made of a lead-free glass having a refractive index of1.8 or more with respect to a wavelength of the image light andcontaining SiO₂ in an amount greater than zero mass % and less than orequal to 35 mass % and TiO₂ in an amount greater than zero mass % andless than or equal to 20 mass %, and the light guide plate has aninternal transmittance of 0.6 or more with respect to a wavelength of400 nm when a plate thickness is 10 mm.
 2. The light guide plate ofclaim 1, wherein the lead-free glass further contains B₂O₃ in an amountgreater than zero mass % and less than or equal to 55 mass %.
 3. Thelight guide plate of claim 1, wherein the lead-free glass furthercontains ZrO₂ in an amount greater than zero mass % and less than orequal to 30 mass %.
 4. The light guide plate of claim 1, wherein thelight guide plate has a first surface and a second surface that faceeach other, and parallelism of the first surface and the second surfaceis 20 arc seconds or less.
 5. The light guide plate of claim 4, whereina difference between a maximum value and a minimum value of a distancefrom the first surface to the second surface is 5 μm or less.
 6. Thelight guide plate of claim 1, wherein when transmitted light of astandard light source D65 when the plate thickness is 10 mm isrepresented by an x-y chromaticity diagram, chromaticity of x is 0.31 to0.34, and chromaticity of y is 0.33 to 0.36.
 7. The light guide plate ofclaim 1, wherein the plate thickness is 0.5 to 1.0 mm.
 8. An imagedisplay device comprising: a light source which outputs illuminationlight; an image display element which receives the illumination lightfrom the light source and outputs image light; and the light guide plateaccording to claim 1 which guides the image light inputted from theimage display element and outputs the image light toward a user's pupil.9. The light guide plate of claim 1, comprising: a first optical elementwhich diffracts the guided image light and outputs the image lighttoward the user's pupil.
 10. The light guide plate of claim 9, whereinthe first optical element is made of a material having a refractiveindex higher than a refractive index of air.
 11. The light guide plateof claim 9, wherein the first optical element is made of a materialhaving a refractive index higher than a refractive index of the lightguide plate.
 12. The light guide plate of claim 9, comprising: a secondoptical element which diffracts the image light inputted from the imagedisplay element and inputs the image light to the light guide plate. 13.The light guide plate of claim 12, wherein the second optical element ismade of a material having a refractive index higher than a refractiveindex of air.
 14. The light guide plate of claim 12, wherein the secondoptical element is made of a material having a refractive index higherthan a refractive index of the light guide plate.
 15. An image displaydevice comprising: a light source which outputs illumination light; animage display element which receives the illumination light from thelight source and outputs image light; and the light guide plateaccording to claim 2 which guides the image light inputted from theimage display element and outputs the image light toward a user's pupil.16. An image display device comprising: a light source which outputsillumination light; an image display element which receives theillumination light from the light source and outputs image light; andthe light guide plate according to claim 3 which guides the image lightinputted from the image display element and outputs the image lighttoward a user's pupil.
 17. An image display device comprising: a lightsource which outputs illumination light; an image display element whichreceives the illumination light from the light source and outputs imagelight; and the light guide plate according to claim 4 which guides theimage light inputted from the image display element and outputs theimage light toward a user's pupil.
 18. An image display devicecomprising: a light source which outputs illumination light; an imagedisplay element which receives the illumination light from the lightsource and outputs image light; and the light guide plate according toclaim 5 which guides the image light inputted from the image displayelement and outputs the image light toward a user's pupil.
 19. An imagedisplay device comprising: a light source which outputs illuminationlight; an image display element which receives the illumination lightfrom the light source and outputs image light; and the light guide plateaccording to claim 6 which guides the image light inputted from theimage display element and outputs the image light toward a user's pupil.20. An image display device comprising: a light source which outputsillumination light; an image display element which receives theillumination light from the light source and outputs image light; andthe light guide plate according to claim 7 which guides the image lightinputted from the image display element and outputs the image lighttoward a user's pupil.